In preparing, for instance, an ophthalmic preparation, nasal preparation or parenteral preparation containing a medicinal compound hardly soluble in water, it is presumable that the drug be suspended in an aqueous medium to give an aqueous suspension. When such aqueous suspension is stored for a long period, the drug occurring as dispersed particles (hereinafter sometimes referred to merely as dispersed particles) tends to undergo aggregation, resulting in increases in size of dispersed particles or sedimentation of dispersed particles and further in secondary aggregation of the dispersed particles that have settled, for example caking. Therefore, efforts have been devoted to prevent the aggregation or sedimentation of dispersed particles as far as possible or, when such aggregation or sedimentation cannot be prevented, to obtain suspensions capable of readily regaining their original state.
One method so far proposed comprises making dispersed particles smaller, decreasing the difference between the specific gravity of dispersed particles with of the dispersion medium and increasing the viscosity of the dispersion medium to thereby prevent the particles from settling. In such cases, for increasing the viscosity of the dispersion medium, the concentration of the suspending agent and/or thickening agent, such as a water-soluble polymer, has generally been selected within the range of 0.2 to 5.0% (w/v: weight/volume).
However, even when the concentration of the suspending agent and/or thickening agent is within such range, the sedimentation of particles cannot entirely be prevented. The problem which remains is that dispersed particles settle and deposit, causing caking, resulting in failure of uniform redispersion.
Another method which is conceivable comprises making drug particles greater in size to thereby improve their redispersibility. In the case of an ophthalmic preparation, however, greater particle sizes may cause a foreign matter sensation or eye irritation upon instillation. In the case of a nasal preparation, greater particle sizes make it impossible to apply it from a spray bottle. In the case of an injection, it is a drawback that it cannot be administered through a needle.